Integrated Refrigerating/Freezing System and Defrost Method

ABSTRACT

A medium and low-temperature integrated refrigeration/freezing system ( 100 ) has the function of discharge gas defrosting. It comprises medium and low-temperature compressors sets ( 120, 122 ) and medium and low-temperature evaporators, as well as control valves ( 141, 142, 143, 144 ), adjusting valves ( 145, 146 ), one-way valves ( 147, 148 ) and expansion valves ( 150, 152, 154 ), and the switching between a refrigerating cycle and the discharge gas defrosting cycle is performed by the combination of actions between multiple control valves. When a first control valve is opened, a second valve is closed and a fourth valve is closed to the refrigerant pipeline to said reservoir, the refrigerating cycle operation is performed; and when the first control valve is closed, the second valve is opened and the fourth valve is closed to the refrigerant pipeline of an intercooler ( 128 ), the discharge gas defrosting operation is performed.

BACKGROUND

This disclosure relates to a refrigerating/freezing display cabinet system and, in particular, to a medium and low-temperature integrated refrigeration/freezing display cabinet system for displaying foods and/or beverage products.

Usually, supermarkets and convenience stores are equipped with various cabinets, and these cabinets can be open type or can have doors, for the presentation of fresh foods or beverages to customers and keeping fresh foods or beverages at certain temperatures. Because the heat-absorbing heat exchanger (“evaporator”) within the refrigerating system frosts when the ambient temperature is close to or below the freezing point of water, the heat transfer efficiency of the heat exchanger falls and even the performance of the whole system falls. The most conventional ways of defrosting include electrical defrosting and discharge gas defrosting. Electrical defrosting is relatively simple but its operational efficiency is relatively low, the time for defrosting is relatively long and the temperature of fresh foods and beverages would rise during defrosting. Discharge gas defrosting is more and more used in refrigerating systems

Generally, all the refrigerating systems comprise at least the following parts: a compressor, a heat rejection heat exchanger (“condenser”), at least one evaporator combined with a display cabinet, an expansion valve and suitable refrigerant pipelines connected with above devices within a closed circulation loop. The expansion valve is provided upstream along the refrigerant pipeline relative to the inlet of the evaporator and is used to expand the liquid refrigerant to a desired lower pressure, wherein the lower pressure is selected according to the specific refrigerant to enter the evaporator. Hereinbelow the detailed description is provided for the refrigerating systems of different temperature levels; FIG. 1 illustrates a block diagram of the structure of a medium-temperature refrigerating system in the prior art and FIG. 2 illustrates a block diagram of the structure of a low-temperature freezing system in the prior art. As illustrated in FIG. 1, the medium-temperature refrigerating system comprises: a compressor 1, a condenser 2, a reservoir 3, an expansion valve 4 and an evaporator 5. When a low-temperature and low-pressure gas, as the refrigerant, is changed into a high-temperature and high-pressure gas after having been compressed by the compressor 1, the high-temperature and high-pressure gas enters the condenser 2 for cooling and becomes a high-temperature and high-pressure liquid accompanied by a process of heat dissipation, and subsequently, the high-temperature and high-pressure liquid flows to the expansion valve 4 after passing through the reservoir 3. As stated above, the expansion valve 4 can select the desired pressure of the refrigerant after expansion according to the selected type of the refrigerant, and the high-temperature and high-pressure liquid is changed into a low-temperature and low-pressure liquid and gas two-phase flow by the expansion valve 4 by throttling, and then after passing through the evaporator 5 such a two-phase flow becomes a low-temperature and low-pressure gas and absorbs heat from the air flow to provide the refrigeration effect. Similarly, in the low-temperature refrigerating system shown in FIG. 2, it also comprises a compressor 11, a condenser 12, a reservoir 13, an expansion valve 14, an evaporator 15 and a liquid injection valve 16. In the refrigerant pipeline portion constituted by the expansion valve 14, the evaporator 15, the compressor 11, the condenser 12 and the reservoir 13, the operational process of the low-temperature refrigerating system is similar to that of the medium-temperature refrigerating system, except that in the low-temperature refrigerating system, in order to lower the discharge temperature of the compressor 11, a branch of the refrigerant pipeline is provided between the compressor 11 and the reservoir 13, and the liquid injection valve 16 is provided in such a branch. When the high-temperature and high-pressure liquid is transformed into the low-temperature and low-pressure gas, heat is absorbed and the suction temperature of the compressor 11 is lowered during this process, and thereby lowering the discharge temperature of the compressor 11 and providing better protection to the compressor set.

SUMMARY

However, in the operation of a system using the independent medium-temperature refrigerating system and low-temperature system, the refrigeration efficiency is relatively low and the consumption of energy is relatively high. To overcome these problems, a medium and low-temperature integrated refrigerating/freezing system has been developed by Carrier Corporation, in which the originally independent medium-temperature refrigerating system and low-temperature system are integrated into a CDU unit, and the operation efficiency of the whole integrated system is improved by the optimized design of the structure and the energy exchange between the two systems. Such an integrated medium and low-temperature refrigerating/freezing system may improve the operational stability of the system, takes less space and may be able to achieve integrated solutions in a “plug and play” manner, thus saving space for installation and tuning for the customers. However, how to apply successfully the discharge gas defrosting (D2D) technology to the integrated medium and low-temperature refrigerating/freezing system becomes a technical problem to be solved by the engineers in the field of refrigeration.

In view of the technical drawbacks in the refrigerating system of the prior art with respect to the integration of a medium-temperature refrigerating system and a low-temperature freezing system, the present disclosure provides a medium and low-temperature integrated refrigeration/freezing system with the function of discharge gas defrosting. The refrigeration/freezing system of the present disclosure may not only improve the refrigeration efficiency and save energy resources, but also can switch between the normal operational state and a discharge gas defrosting state, which may improve the operational stability of the system.

According to one aspect of the present disclosure, there is provided a medium and low-temperature integrated refrigeration/freezing system with the function of discharge gas defrosting. A medium-temperature refrigerating system and a low-temperature freezing system are integrated to apply D2D technology. The system comprises at least a medium-temperature compressor, a low-temperature compressor, a condenser, a reservoir, an intercooler, a medium-temperature evaporator, a low-temperature evaporator, and four control valves, two adjusting valves, two one-way valves and three expansion valves; and by controlling the combination of actions between these four control valves, the switching is performed between the refrigerating cycle operation and the discharge gas defrosting operation

When the first control valve is opened and the second valve is closed, and the fourth valve is closed to the refrigerant pipeline to the reservoir, the medium and low-temperature integrated system may perform the refrigerating cycle operation. Further, when the system performs the refrigerating cycle operation, the suction temperature of the low-temperature compressor may be adjusted by the first adjusting valve, so as to lower the discharge temperature of the low-temperature compressor.

When the first control valve is closed and the second valve is opened, and the fourth valve is closed to the refrigerant pipeline to the intercooler, the system may perform the discharge gas defrosting operation. Further, when the system performs the discharge gas defrosting operation, the suction temperature of the low-temperature compressor may be adjusted by the second adjusting valve, so as to lower the discharge temperature of the low-temperature compressor.

When the system performs the refrigerating cycle operation or the discharge gas defrosting operation, the medium-temperature compressor and the low-temperature compressor may be both in operation.

According to another aspect of the present disclosure, there is provided a method for switching between the refrigerating cycle operation and the discharge gas defrosting operation in a medium and low-temperature integrated refrigeration/freezing system. The medium and low-temperature integrated refrigeration/freezing system comprises at least a medium-temperature compressor, a low-temperature compressor, a condenser, a reservoir, an intercooler, a medium-temperature evaporator, a low-temperature evaporator, and four control valves, two adjusting valves, two one-way valves and three expansion valves, and by using the combination of actions between these four control valves the switching between the refrigerating cycle operation and the discharge gas defrosting operation is performed. More particularly, when the first control valve is opened and the second valve is closed, and the fourth valve is closed to the refrigerant pipeline to the reservoir, the medium and low-temperature integrated system performs the refrigerating cycle operation; and when the first control valve is closed and the second valve is opened, and the fourth valve is closed to the refrigerant pipeline to the intercooler, the medium and low-temperature integrated system performs the discharge gas defrosting operation.

When the system performs the refrigerating cycle operation, the suction temperature of the low-temperature compressor may be adjusted by the first adjusting valve, so as to lower the discharge temperature of the low-temperature compressor.

When the system performs the discharge gas defrosting operation, the suction temperature of the low-temperature compressor may be adjusted by the second adjusting valve, so as to lower the discharge temperature of the low-temperature compressor.

The low-temperature evaporator may be provided with a temperature sensor, and the relevant parameters of said temperature sensor may be preset to determine the starting time and ending time of the discharge gas defrosting operation.

By using of the medium and low-temperature integrated refrigeration/freezing system of the present disclosure, it not only can perform normal refrigerating cycle operation and discharge gas defrosting operation based on a medium-temperature compressor set and a low-temperature compressor set, but also it can improve the operational efficiency of the whole integrated system by using the heat exchange between the medium-temperature refrigerating system and the low-temperature freezing system. Additionally, a temperature sensor may be fitted in the low-temperature evaporator and the starting time and ending time of the discharge gas defrosting can be determined quickly through the intelligent control of relevant parameters. After the low-temperature evaporator is optimized by redesign, the defrosting time can be reduced and the defrosting can be performed thoroughly.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present disclosure will be more apparent to readers after reading the embodiments of the present disclosure with reference to the accompanied drawings, in which:

FIG. 1 is a block diagram of the principles of the structure of a medium-temperature refrigerating system in the prior art;

FIG. 2 is a block diagram of the principles of the structure of a low-temperature freezing system in the prior art;

FIG. 3A shows a schematic diagram of the structure of a discharge gas defrosting system in performing normal refrigerating cycle, and FIG. 3B shows a schematic diagram of the structure of the discharge gas defrosting system in performing discharge gas defrosting;

FIG. 4 shows a schematic diagram of the structure of a medium and low-temperature integrated refrigeration/freezing system;

FIG. 5 shows a schematic diagram of the structure of a medium and low-temperature integrated refrigeration/freezing system with the function of discharge gas defrosting according to one or more aspects of the present disclosure;

FIG. 6 shows a schematic diagram of the principles of the medium and low-temperature integrated refrigeration/freezing system shown in FIG. 5 in a normal operational state; and

FIG. 7 shows a schematic diagram of the principles of the medium and low-temperature integrated refrigeration/freezing system shown in FIG. 5 in a discharge gas defrosting state.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure shall be described in more detail with reference to the accompanied drawings.

FIG. 3A shows a schematic structural diagram of a discharge gas defrosting system in performing normal refrigerating cycle and FIG. 3B shows a schematic structural diagram of a discharge gas defrosting system in performing discharge gas defrosting. Referring to FIGS. 3A and 3B, the discharge gas defrosting system mainly comprises a medium-temperature compressor 21, a low-temperature compressor 22, a medium-temperature evaporator 31, a low-temperature evaporator 30 and control valves 41 to 45. In FIGS. 3A and 3B, the parts shown by respective dash lines represent the refrigerant pipelines not involved in the cycles of the respective operational states, while the parts shown by respective solid lines represent the refrigerant pipelines involved in the cycles of the respective operational states. It can be seen in FIG. 3A that when the system is performing the normal refrigerating cycle, after the low-temperature and low-pressure refrigerant has been compressed by the medium-temperature compressor and the low-temperature compressor, the control valve 41 is open and the high-temperature and high-pressure gas discharged from the medium-temperature compressor 21 and the low-temperature compressor 22 is transmitted to the condenser (not shown) and then returns to respective compressors via the medium-temperature evaporator 31 and the low-temperature evaporator 30.

After the high-temperature and high-pressure liquid is transformed into low-temperature and low-pressure gas by the low-temperature evaporator 30, the suction temperature of the low-temperature compressor 22 is adjusted by the control valve 44 by throttling, thereby the discharge temperature of the low-temperature compressor 22 is lowered and the compressor 22 is better protected.

It can be seen in FIG. 3B that when the system performs the discharge gas defrosting operation, the control valve 41 is shut (to block flow to the condenser) and the low-temperature and low-pressure refrigerant is changed into the high-temperature and high-pressure gas after having been processed by the medium-temperature compressor 21 and low-temperature compressor 22, enters the low-temperature evaporator via the control valves 42 and 43, is changed into the high-temperature and high-pressure liquid after having condensed in parts of the refrigerant pipelines of the system, and finally returns via the medium-temperature evaporator to the medium-temperature compressor and to the low-temperature compressor (via the control valves 44 and 45). It should be understood by those skilled in the art that an expansion valve can also be added at the intake of the medium-temperature evaporator 31 to change the high-temperature and high-pressure liquid into a low-temperature and a low-pressure liquid and gas two-phase flow by throttling, which changes into the low-temperature and low-pressure gas after having passed through the evaporator and absorbs heat from the air flow to produce the refrigeration effects.

FIG. 4 shows a schematic structural diagram of a medium and low-temperature integrated refrigeration/freezing system. Referring to FIG. 4, in the medium and low-temperature integrated refrigeration/freezing system, the original independent medium-temperature system and low-temperature system of FIGS. 1 and 2 are integrated into a CDU unit 20, in which the performance of the integrated system is improved by the heat exchange between the medium-temperature system and the low-temperature system. The integrated system mainly comprises: a medium-temperature compressor 21, a low-temperature compressor 22, a heat rejection heat exchanger (condenser) 23, a reservoir 24, an expansion device (valve) 25, an intercooler 26, an adjusting valve 27, expansion devices (valves) 28 and 29, a low-temperature heat absorption heat exchanger (evaporator) 30 and a medium-temperature evaporator 31. Furthermore, the condenser 23 combines the cooling function of the condenser in the independent medium-temperature system and that of the condenser in the independent low-temperature system, and the reservoir 24 replaces the respective reservoirs of the independent medium-temperature system and of the independent low-temperature system. The intercooler 26 and the expansion valve 25 are used to adjust the subcooling at the low temperature to improve the overall performance of the system. Although shown connected to a single medium temperature evaporator and cabinet and a single low temperature evaporator and cabinet, a given CDU may be connected to multiple such medium temperature evaporators and/or cabinets and/or multiple low temperature evaporators and/or cabinets. Also, each CDU may have multiple such medium temperature compressors and/or low temperature compressors in respective compressor sets.

A brief introduction to the operational principles of the integrated system is: the low-temperature and low-pressure refrigerant is changed into a high-temperature and high-pressure gas after having been compressed by the medium-temperature compressor 21 and the low-temperature compressor 22; the high-temperature and high-pressure gas changes into high-temperature and high-pressure liquid after having entered the condenser 23 and dissipated a large quantity of heat. After passing through the reservoir 24, the refrigerant is divided into three pipelines or branches: in the first pipeline 51 it is changed into a low-temperature and low-pressure liquid and gas two-phase flow by throttling by the expansion valve 29, is changed into a low-temperature and low-pressure gas after having entered the medium-temperature evaporator 31 and absorbed heat to provide the refrigeration effects, and then returns to the medium-temperature compressor 21; in the second pipeline 52 it is changed into a low-temperature and low-pressure liquid and gas two-phase flow by precooling by the intercooler 26 and throttling by the expansion valve 28, changes into a low-temperature and low-pressure gas after having entered the low-temperature evaporator 30 and absorbed heat to produce the refrigeration effects, and then returns to the low-temperature compressor 22; and in the third pipeline/branch 54 it passes through the expansion valve 25 and the intercooler 26 (where it absorbs heat from the flow in the second pipeline 52) to adjust the subcooling at the low temperature, and the refrigerant returns to the medium-temperature compressor 21 after having been subcooled, and then returns to the low-temperature compressor 22 after the suction temperature at the low-temperature level is adjusted via the adjusting valve 27 which sets the balance of flow between branches (sub-branches) 54-1 and 54-2. It can be seen that in the integrated system, by using the subcooled liquid-supplying temperature at the low-temperature level at the intermediate heat exchanger (the intercooler 26), the efficiency of the low-temperature level is improved by the energy transfer. It is shown by experiment data that the energy efficiency ratio of the low-temperature level is 1.1 and that of the medium-temperature level is 2.2. Thus, the performance of the whole integrated system and the operational stability of the medium and low-temperature systems can be improved by the heat exchange between the medium-temperature system and the low-temperature system.

FIG. 5 shows a schematic structural diagram of a medium and low-temperature integrated refrigeration/freezing system 100 with the function of discharge gas defrosting and having a CDU 102 coupled to a low temperature cabinet 202 and a medium temperature cabinet 200. Referring to FIG. 5, in comparison with the medium and low-temperature integrated refrigeration/freezing system shown in FIG. 4, the medium and low-temperature integrated refrigeration/freezing system 100 with the function of discharge gas defrosting of the present disclosure introduces an operational process for discharge gas defrosting and it can ensure the switching between the normal operational state and the discharge gas defrosting state in the medium and low-temperature integrated system by controlling relevant valves (under control of a control system (e.g., a microcontroller) (not shown)).

Particularly, the integrated system shown in FIG. 5 mainly comprises: a medium-temperature compressor 120, a low-temperature compressor 122, a condenser 124, a reservoir 126, an intercooler 128, a medium-temperature evaporator 130, a low-temperature evaporator 132, control valves 141-144, adjusting valves 145 and 146, one-way valves (check valves) 147 and 148, and expansion devices (valves) 150, 152, 154.

The condenser combines simultaneously both the cooling functions of the condenser in the independent medium-temperature system and the condenser in the independent low-temperature system, and the reservoir has replaced the respective reservoirs of the independent medium-temperature system and of the independent low-temperature system. The switching between the normal operational state and the discharge gas defrosting state can be realized by controlling the combination of actions between control valves 141-144, and the suction status of the low-temperature system in the normal operational state and the discharge gas defrosting state can be adjusted by the adjusting valves 145 and 146, so as to lower the suction temperature of the low-temperature compressor, and to lower the discharge temperature of the low-temperature compressor to better ensure the stable operation of the low-temperature compressor set.

FIG. 6 shows a schematic diagram of the principles of the medium and low-temperature integrated refrigeration/freezing system shown in FIG. 5 in its normal operational state. Here, the parts shown by the dashed lines in FIG. 6 represent the refrigerant pipelines not involved in circulation during the normal operation, while the parts shown by the solid lines represent the refrigerant pipelines involved in the circulation during the normal operation. The system has pipelines (branches) 160, 162, and 164 (including sub-branches 164-1 and 164-2) that are similarly configured and perform similar functions to the corresponding pipelines or branches 50, 52, 54 of FIG. 4. An additional pipeline/branch 166 is provided between a location along the main flowpath 167 downstream of the condenser 22 and upstream of the intercooler 26 to the pipeline/branch 162 downstream of the intercooler at the control valve 144 for recirculating refrigerant in the defrost mode (discussed below).

Similarly, a pipeline/branch 168 is provided from a location downstream of the compressors 120 and 122 to upstream of the low temperature compressor 122. The control valves 142 and 143 are respectively positioned along the pipeline/branch 168 for diverting compressed refrigerant in the defrost mode (discussed below). The adjusting valve 146 is along a bypass pipeline/flowpath 170 joining the suction conditions of the two compressors. The one-way valve 147 is along a bypass pipeline/flowpath 172 in parallel with the expansion valve 152.

When the integrated system 100 is in the normal operational state, the control valve 142 is set (shut off) to prevent flow along the pipeline/flowpath branch 168 and the one-way valve 147 is in a closed condition, and the control valve 14 is set to block flow along the pipeline/branch flowpath 166 while permitting flow along the pipeline/branch 162 from the intercooler 128 to the expansion valve 152 and low temperature evaporator 132. The low-temperature and low-pressure refrigerant is changed into the high-temperature and high-pressure gas after having been compressed by the medium-temperature compressor 120 and the low-temperature compressor 122, the high-temperature and high-pressure gas enters the condenser 124 via the control valve 141 for cooling and becomes a high-temperature and high-pressure liquid accompanied by a process of heat dissipation; then the high-temperature and high-pressure liquid is divided into three pipelines 160, 162, 164 after having passed through the one-way valve 148 and the reservoir 126, and the detailed operations of the three pipelines have been described above with respect to FIG. 4 and shall not be further repeated here.

FIG. 7 shows a schematic diagram of the medium and low-temperature integrated refrigeration/freezing system of FIG. 5 in the discharge gas defrosting state. Similarly, in FIG. 7, parts shown by the dash lines represent the refrigerant pipelines not involved in the circulation during the discharge gas defrosting, while the parts shown by the solid lines represent the refrigerant pipelines involved in the circulation during the discharge gas defrosting. Particularly, when the integrated system is in the discharge gas defrosting state, the control valve 141 is shut off/closed, the one-way valve 148 is in a closed condition and the expansion valves 152 and 154 are shut off/closed, the control valve 142 is open (to permit flow along the branch 168) and the one-way valve 147 is in an open condition.

Referring to FIG. 7, the system's operation principles during the discharge gas defrosting is also described according to the refrigerant's direction of flow: the low-temperature and low-pressure refrigerant, exits the medium temperature evaporator 130 and enters into the medium-temperature compressor 120 and, via the adjusting valve 146, into the low-temperature compressor. The refrigerant is compressed, by the compressors, is changed into the high-temperature and high-pressure gas, enters the low-temperature evaporator 132 (in a reverse of the refrigerant direction) via the control valves 142 and 143. The refrigerant then enters the expansion valve 150 via the one-way valve 147 (bypassing the expansion valve), the control valve 144 and the reservoir 126, and returns to the compressor set via the medium-temperature evaporator 130 after having been transformed into a low-temperature and low-pressure liquid and gas two-phase flow by throttling in the expansion valve 150. During the process of discharge gas defrosting, the use of a condenser is not involved in any one of the refrigerant pipelines. To further improve the efficiency of the discharge gas defrosting and to reduce the time of the discharge gas defrosting, a temperature sensor (not shown) may be embedded in the low-temperature evaporator and the starting time and ending time of the discharge gas defrosting can be determined quickly by the intelligent control of relevant parameters by the control system (controller). In a reengineering from the FIG. 4 baseline, after the low-temperature evaporator is optimized by redesign, the time of defrosting can be reduced and the defrosting can be performed thoroughly.

It should be understood by a person skilled in the art that the above-mentioned compressors, medium-temperature compressors and low-temperature compressors are also applicable to compressor sets, medium-temperature compressor sets and low-temperature compressor sets. Therefore, the embodiments according to one or more aspects of the present disclosure are not limited to the cases of a compressor, a medium-temperature compressor and a low-temperature compressor.

The embodiments of the present disclosure are described above with reference to the accompanied drawings. However, modifications and variations of the embodiment can be made by those skilled in the art without departing from the spirit and scope of the present disclosures. And these modifications and variations fall within the scope of the present disclosure as defined in the claims.

Although an embodiment is described above in detail, such description is not intended for limiting the scope of the present disclosure. It will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, when implemented in the reengineering of an existing container configuration, details of the existing configuration may influence or dictate details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims. 

1. A medium and low-temperature integrated refrigeration/freezing system (100), comprising: a medium-temperature compressor (120), a low-temperature compressor (122), a condenser (124), a reservoir (126), an intermediate heat exchanger (128), a medium-temperature evaporator (130), and a low-temperature evaporator (132), characterized in that said system further comprises: a first control valve (141); a second control valve (142); a third control valve (143); a fourth control valve (144); a first adjusting valve (145); a second adjusting valve (146); a first one-way valve (147); a second one-way valve (148); a first expansion valve (150); a second expansion valve (152); and a third expansion valve (154), which are for switching between a refrigerating cycle operation and a discharge gas defrosting cycle operation by controlling the combination of actions between said first, second, third and fourth control valves.
 2. The medium and low-temperature integrated refrigeration/freezing system as claimed in claim 1, characterized in that when said first control valve (141) is opened, said second control valve (142) is closed, and said fourth control valve (144) is closed to the refrigerant pipeline of the reservoir, said system performs the refrigerating cycle operation.
 3. The medium and low-temperature integrated refrigeration/freezing system as claimed in claim 2, characterized in that when said system performs the refrigerating cycle operation, the suction temperature of the low-temperature compressor is adjusted by said first adjusting valve (145) to lower the discharge temperature of said low-temperature compressor.
 4. The medium and low-temperature integrated refrigeration/freezing system as claimed in claim 1, characterized in that when said first control valve (141) and said second control valve (142) are opened, and said fourth control valve (144) is closed to the refrigerant pipeline of the intercooler, said system performs the discharge gas defrosting operation.
 5. The medium and low-temperature integrated refrigeration/freezing system as claimed in claim 4, characterized in that when said system performs the discharge gas defrosting operation, the suction temperature of said low-temperature compressor is adjusted by said second adjusting valve (146), so as to lower said discharge temperature of the low-temperature compressor.
 6. The medium and low-temperature integrated refrigeration/freezing system as claimed in claim 1, characterized in that when said system performs the refrigerating cycle operation or the discharge gas defrosting operation, said medium-temperature compressor and the low-temperature compressor are both in operation.
 7. The medium and low-temperature integrated refrigeration/freezing system as claimed in claim 4, characterized in that said low-temperature evaporator is provided with a temperature sensor, and relevant parameters of said temperature sensor are preset to determine the starting time and ending time of the discharge gas defrosting operation.
 8. The medium and low-temperature integrated refrigeration/freezing system as claimed in claim 1, characterized in that said medium-temperature compressor can be a medium-temperature compressor set, said low-temperature compressor can be a low-temperature compressor set; said medium-temperature evaporator can be a group of medium-temperature evaporators, and said low-temperature evaporator can be a group of low-temperature evaporators.
 9. A method for switching between a refrigerating cycle operation and a discharge gas defrosting operation in a medium and low-temperature integrated refrigeration/freezing system, said medium and low-temperature integrated refrigeration/freezing system comprising at least a medium-temperature compressor, a low-temperature compressor, a condenser, a reservoir, an intercooler, a medium-temperature evaporator, a low-temperature evaporator, and a first control valve, a second control valve, a third control valve, a fourth control valve, a first adjusting valve, a second adjusting valve, a first one-way valve, a second one-way valve, a first expansion valve, a second expansion valve and a third expansion valve, characterized in that: when said first control valve is opened and said second valve is closed, and said fourth valve is closed to the refrigerant pipeline of said reservoir, the system performs the discharge gas defrosting operation; and when said first control valve is closed and said second valve is opened, and said fourth valve is closed to the refrigerant pipeline of the intercooler, said system performs the discharge gas defrosting operation.
 10. A switching method as claimed in claim 9, characterized in that when said system performs the refrigerating cycle operation, the suction temperature of the low-temperature compressor is adjusted by said first adjusting valve, so as to lower the discharge temperature of said low-temperature compressor.
 11. The switching method as claimed in claim 9, characterized in that when said system performs the discharge gas defrosting operation, the suction temperature of the low-temperature compressor is adjusted by said second adjusting valve, so as to lower the discharge temperature of said low-temperature compressor.
 12. The switching method as claimed in claim 9, characterized in that said low-temperature evaporator is provided with a temperature sensor, and the relevant parameters of said temperature sensor are preset to determine the starting time and ending time of the discharge gas defrosting operation.
 13. The switching method as claimed in claim 9, characterized in that said medium-temperature compressor can be a medium-temperature compressor set, said low-temperature compressor can be a low-temperature compressor set; said medium-temperature evaporator can be a medium-temperature evaporator set, and said low-temperature evaporator can be a low-temperature evaporator set.
 14. A refrigeration system (100) comprising: a first compartment (200); a second compartment (202); at least one compressor (120,122) for compressing refrigerant; a heat rejection heat exchanger (124) for cooling compressed refrigerant from the at least one compressor; a reservoir (126) for storing condensed refrigerant; a first expansion device (150) and a first evaporator(130) in a flow path associated with the first compartment (200) for expanding refrigerant and cooling the first compartment; and a second expansion device (152) and second evaporator (132) in a flow path associated with the second compartment for expanding the cooled refrigerant and cooling the second compartment, further characterized by: means for providing: a normal refrigerating condition in which refrigerant from the compressors is cooled by the heat rejection heat exchanger, passed to the evaporators to absorb heat from the compartments, and returns to the at least one compressor; and a defrost mode wherein: the heat rejection heat exchanger is bypassed so that compressed refrigerant is passed to the second evaporator in a reverse direction to the refrigerating mode to heat to deliver heat to the second compartment; after heating the second compartment, the refrigerant passes to the first expansion device and first evaporator.
 15. The system of claim 14 wherein: the heat rejection heat exchanger and the reservoir are common to the flow path associated with the first compartment and the flow path associated with the second compartment.
 16. A method for operating the system of claim 14, comprising: operating in the refrigeration mode; and switching to operation in the defrost mode.
 17. The method of claim 16, wherein: in the defrost mode, the first evaporator continues to cool the first compartment. 